Indication on a hearing aid to let the user know whether the hearing aid is for the left or for the right ear is usually provided on hearing aids of the kind for wearing behind the ear (BTE). This indication can be in the form of a coloured “dot” having one colour for left and another for the right ear hearing aid. The dot can be difficult to spot for especially elderly users of hearing aids. Further the dot is provided during production and when the dot is applied the hearing aid is dedicated to being either left or right. It would be an advantage if the left/right choice of the hearing aid could be made at a later state, eg during or just prior to fitting the hearing aid to an individual. Further it should be noted that the marking is only needed by the user when the hearing aid is to be positioned on or behind the ear, at other times the marking has no function. Thus a marking is desired, which is only visible when needed, and which may be applied during or right prior to the customarysation of the hearing aid to the individual user.